For many years it was generally believed that the production of reactive oxygen species (ROS) was an unwanted byproduct of aerobic metabolism and other cellular enzymatic processes and that ROS were uniformly deleterious in nature. Since our last review, we have spent the majority of our energies pursuing an alternative hypothesis; that production of ROS are tightly regulated, the targets of ROS are specific, and that oxidants contribute to disease progression, at least in part, through the redox-regulation of specific pathways. To pursue this overall hypothesis we have attempted to identify the sources of ROS and their specific targets within cells. Regarding the targets of ROS, since the last review, we have developed a general proteomic-based method to detect oxidatively-modified proteins within cells. We have also taken a more directed approach and analyzed how ROS specifically modify the dual specific phosphatase and cell cycle regulator, Cdc25C. These results provide general insights as to how ROS specifically modify protein targets. We have also analyzed the sources of oxidants in vivo especially within the context of the vessel wall. This work is continuing with our analysis of mice recently generated within our laboratory that lack the gene for xanthine oxidoreductase. Besides identifying the sources and targets of ROS, we have also pursued how oxidants contribute to disease. A recent, primary focus has been the role of oxidants in aging. Since previous experimental evidence suggested that mitochondria-derived oxidants played an important role in aging, much of our efforts have been centered on this organelle. We established that similar to cytosolic-derived ROS, oxidants originating from the mitochondria can also play a role in cell signaling. We have also probed the role of mitochondria in cellular life span and what might limit the mitochondrial biogenesis program in cells. Since mitochondrial uncoupling is believed to regulate ROS production, we have begun an analysis of the in vivo physiological role of these important proteins. Finally, we have begun to analyze how specific redox-dependent signaling pathways may contribute to aging. Although the work on oxidants represents our primary focus, a small amount of time and energy is devoted to specific translational efforts. As an extension of our work on aging, we became interested in understanding why atherosclerotic disease incidence increases so markedly with age and whether common mechanism may underlie organismal aging and age-related diseases. To this end we have shown that certain atherosclerotic risk factors can accelerate endothelial senescence. More recently, we have demonstrated that the depletion or exhaustion of circulating endothelial progenitor cells may contribute to vascular disease. The latter study has spurred an interest in stem cell biology that is continuing in the laboratory.